This invention relates to a signal transmission apparatus for a steering control board which is used for transmitting signals between the control board mounted at the central portion of a steering wheel in a floating relation with respect to the wheel and a stationary control unit mounted on a vehicle body.
In vehicles, a steering wheel is positioned nearest to the driver and moreover near hands of the driver. Thus, in order to improve operability, a control board equipped with key switches, etc. used in the control and indication for load-vehicle units and devices is preferable to be mounted at the central portion of the steering wheel.
However, since a steering mechanism adapted to transmit the rotational movement of the steering wheel to a steering shaft has a complicated structure, it is difficult to wire signal cables connecting the control board mounted at the central portion of the steering wheel (referred to as steering control board) with a stationary control unit. In particular, when the control board is mounted on the steering wheel in a floating relation to ensure that the former remains motionless even with the latter being operated to turn, a still more complicated mechanism must be interposed between the control board and the steering wheel. This makes it very difficult to connect the control board with the stationary control unit using the normal lines.
U.S. patent application Ser. No. 399,582 filed on July 17, 1982, which is assigned to the same assignee as the present application, discloses a system in which the control board is connected with the stationary control unit using a slip ring and a brush, so that transmission of both electric power and signals is effected through those two members in slide-contact with each other. According to this system, electric power supplied to the control board as well as many data generated from the control board can be transmitted without using a number of lines.
But, since the proposed system employs mechanical elements such as a slip ring and a brush, it is hard to ensure satisfactorious reliability for a long term. For example, if the contact surfaces of the slip ring and the brush are oxidized attendant on changes with aging, or if a pressing force urged upon the contact portions is lowered, the circuit may be disconnected temporarily due to mechanical vibrations or so from the external. As to transmission of electric power, it is possible that temporary disconnection of the power supply circuit is made to cause no adverse influence by using a capacitor with relatively large capacity or by other techniques. As to transmission of signals, on the other hand, there will be such a possible danger that an error is caused in the received information and hence the stationary control unit malfunctions.
To eliminate such adverse influence, there has been proposed another system which utilizes a light for transmission of signals. In this system, however, the relative position between an optical element (e.g., a light emitting element) on the steering wheel and an optical element (e.g., a light receiving element) on the stationary load-vehicle side is varied with the steering wheel being turned for steering operation. Thus, transmission of signals will be interrupted temporarily.
To eliminate the foregoing drawback, there has been also proposed such a system that a number of light emitting elements are disposed at the central portion of the steering wheel in the annular form and a light receiving element is disposed at a position opposite to those light emitting elements, whereby a light from one of the light emitting elements can always reach the light receiving element. With this system, however, in case the control board is mounted on the steering wheel in a floating relation, spokes for supporting the steering wheel must be arranged between the light emitting and receiving elements. Thus, when the steering wheel is turned, the optical path between both elements will be interrupted by the presence of the spokes. Further, this system requires a number of light emitting elements, so that it is not practically advantageous in point of the cost and power consumption.